Plasma generating apparatus having an arc restricting region



rim-m1 HU'UM R. D. BUHLER Jan. 2, 1968 PLASMA GENERATING APPARATUSHAVING AN ARC RESTRICTING REGION 2 Sheets-Sheet 1 Filed April 22, 1963ROLF D. BUHLER INVENTOR.

MUN DOW .rZmNNDU ATTO R N EYS.

2 Sheets-Sheet 2 R. D. BUHLER PLASMA GENERATING APPARATUS HAVING AN ARCRESTRICTING REGION D. BUHLEIZ INVENTOR.

ATTORNEYS.

Jan. 2, 1968 Filed April 22, 1963 MUMDOW PZMQNDU 3,361,927 FLASMAGENERATING APPARATUS HAVING AN ARC RESTRICTING REGION Rolf D. Buhler,Newport Beach, Calif., assignor, by mesne assignments, to GianniniScientific Corporation, Amityvillc, N.Y., a corporation of DelawareFiled Apr. 22, 1963, Ser. No. 274,616 3 Claims. (Cl. 315-411) Thisinvention pertains to the generation of plasma, and in particular togenerating plasma of high enthalpy under high pressure conditions.

While the arrangement of this invention possesses utility where ever thegeneration of plasma is desired, it has been found especially useful inproducing plasma of high heat content where the pressures are large.There are a variety of uses for such plasma generation, such as inresearch for simulating the re-entry of a nose cone into the earthsatmosphere. The great velocity of a nose cone at such conditions meansthat it encounters considerable pressures from the resisting atmosphere.Also the air friction generates extremely high temperatures.Consequently, in certain testing programs the need arises for plasmareproducing pressures and heats of such magnitude.

Several problems have been encountered in efforts to create highenthalpy plasma at comparable pressures.

The current density of the arc increases to an extent that it becomesdifficult to mix the gas with the are. This reduces the efilciency ofthe heat transfer between the arc and the gas. Moreover, the highcurrent densities cause the terminal portions of the are to tend to burninto the electrodes and melt them. The result is electrode damage andcontamination of the plasma by electrode material. Additionally, the arcbecomes less stable under higher pressure conditions. Starting the arebecomesquite difiicult. Heat losses by convection and radiation to thewalls of the chamber, and hence the cooling water, become much largerunder such conditions.

The present invention provides a plasma generator that is capable ofproducing'high enthalpy plasma at elevated pressures It includes twoannular electrodes spaced apart in a chamber having a relatively narrowinterconnecting passageway. The are is constricted to this passagewaythrough which flows the gas which is introduced into the chamber in arotational pattern. Consequently, the gas and arc are confined to thesame locality and there is a more eflicient transfer of heat from thearc to the gas. The footpoints of the are are rotated which avoidsburning of the electrodes and extends electrode life.

Accordingly, it is an object of this invention to provide a plasmagenerator capable of producing high enthalpy plasma at high pressures.

Another object of this invention is to provide a reliable and stablearrangement for the generation of plasma, adapted for various purposesincluding those where high heat content and elevated pressures areneeded.

A further object of this invention is to provide a plasma generatingarrangement in which the arc is restricted to a relatively narrow paththrough which the gas also passes to become efliciently heated by theare.

A still further object of this invention is to provide a plasmagenerating arrangement in which both terminals of the are are moved.

Yet another object of this invention is to provide a plasma generatingarrangement in which the arc is relatively short to minimize loss ofpower due to convection and radiation.

These and other objects will become apparent from the following detaileddescription taken in connection with the accompanying drawing in which:

FIGURE 1 is a longitudinal sectional view of a plasma generating deviceembodying the invention,

3,361,927 Patented Jan. 2, 1968 FIGURE 2 is a transverse sectional viewtaken along line 22 of FIGURE 1, and

FIGURE 3 is a longitudinal sectional view similar to FIGURE 1 of amodified form of the invention.

With particular reference to FIGURES 1 and 2 of the drawing, the plasmagenerating arrangement of this invention includes an elongated housing5, This is constructed of a refractory material resistant to extremetemperatures, such as tantalum carbide or hafnium carbide. The member 5includes an elongated relatively narrow opening 6 at its axis. Thehousing 5 and the aperture 6 may be cylindrical.

At the left-hand end of the housing, as illustrated, a generallycup-shaped member 7 fits on to the housing cooperating with the housingto define a chamber 8. The end section 7 also is of heat resistantmaterial such as one of the high temperature ceramics named above. Asmoothly convergent wall 9 connects the enlarged chamber 8 with thesmaller axial opening 6.

The gas is introduced into the unit adjacent the outer wall 10 of theend member 7. It is conducted to the chamber 8 through a conduit 11 anddischarges inside the chamber tangentially along the cylindrical wall 12of the chamber. In this manner the gas, which may be at a relativelyhigh pressure, is caused to rotate rapidly and smoothly as it enters thechamber 8.

vAt the opposite, or right-hand .end, of the housing 5 are locatedannular members 13 and '14 which together with the housing define aplenum or mixing chamber 15. The member 13, like the housing 5 and theend member 7, is made of a heat resistant ceramic. The member 14includes an outwardly convergent inner wall 16 leading to the dischargeopening 17 for the unit. Normally member 14 will be cooled in view ofthe rush of hot gases. along the wall 16 toward the opening 17 when theunit is in operation. Consequently, the end member 14 may be constructedof a suitable metal and made hollow to define an annular passageway 18around the wall 16. Cooling water enters the chamber 18 through an inletline 19 being exhausted through return line 20 that connects to thepassageway 18 at a location opposite from the line 19.

The front and rear electrodes 22 and 23 are disposed within the chambers8' and 15, respectively. These are annular members coaxial with thehousing and end chambers, and may include internal passageways wherebythe electrodes are suitably water cooled. It can be seen that theelectrodes 22 and 23 have inside diameters that are considerably largerthan the diameter of the elongated opening 6 that defines the gapbetween the electrodes.

Electrical energy for the plasma generating arrangement of thisinvention is supplied by an appropriate current source 25. This may be asource of direct current, or it may be alternating current of single ormultiphase. Lead 26 from the current source 25 connects to a coil 27wound around the exterior of the chamber 8. This coil may be made up ofa length of copper tubing embedded in a block 28 of ceramic or otherheat resistant material. The hollow tubing allows water to be circulatedthrough the coil to keep it cooled during operation of the unit of thisinvention. A relatively short lead 29 connects the coil 27 to theelectrode 22 which thereby is in series with the coil.

Conductor 31 interconnects the other terminal of the current source 25and a similar coil 32 circumscribing the mixing chamber 15. This coilthrough lead 33 connects in turn to the front electrode 23,

In operation of the device of this invention gas under a relatively lowpressure is admitted into the chamber 8 through the inlet line 11. Atorch 35, which may comprise a small plasma generator, is ignited atthis time thereby ionizing the gas that passes through the housing 5 andinto the plenum chamber 15. Then current is passed through theconductors 26 and 31 and the are 36 is struck between the electrodes 22and 23. As the term arc is used herein it refers to the central thinfilament represented by the line 36, and to the high temperature gas orplasma that immediately surrounds the filament. After the arc isinitiated the gas pressure is increased to its full value.

The coils 27 and 32 generate a magnetic field traveling generally asindicated by the dotted lines 37 in FIGURE 1 of the drawing. It may beobserved that the are 36 in extending downwardly to enter the centralpassageway 6 through the housing 5 cuts the lines of flux in theopposite direction from the other portion of the are at the opposite endthat moves upwardly from the passageway 6 to the terminal 23. As the arcpasses through the flux field at either end it is caused to rotate inaccordance with the Lorentz forces encountered. Thus, the arc travelsalong the inner wall of each electrode and does not remain stationary atits footpoints. In this embodiment of the invention the opposite ends ofthe arc rotate in different directions by virtue of the fact that thefootpoints of the are cut the flux field in opposite directions at thetwo terminals.

The rotational flow of the gas at the chamber 8 adjacent the terminal 22assists in the rotation of the are at that location. This gas normallyis admitted at high pressures, such as 600 p.s.i.a. The entry velocitymay be sonic as the gas leaves the tube ll. Thus, the footpoint of theare at the terminal 22 is caused to rotate both magnetically and fromthe force of the gas impinging on it. Various gases may be usedincluding nitrogen, hydrogen, argon and helium, as well asoxygen-containing gases such as carbon monoxide.

As a result of this construction, the plasma generator of this inventionis an efficient unit producing plasma of high enthalpy. The power sourcetypically will deliver around one million watts from one thousandarnperes at one thousand volts. This is true under conditions where thegas may be introduced into the chamber 8 under very high pressures. Therotational path of the gas within the housing as it flows from thechamber 3 toward the outlet 17 causes a thorough mixing and thetransmission of a large amount of heat from the arc to the gas. Therotation of the footpoints of the arc avoids erosion and melting of theelectrodes, increasing the life of the electrodes and reducingcontamination of the output plasma. The fact that the arc is forced tofollow the relatively narrow passageway 6 at the axis of the unit aidsconsiderably in the transmission of heat to the gas. The gas also flowsthrough this narrow opening so that the arc and the gas are confined tothe same locality. The vortical flow of the gas as it passes through thepassageway 6 assures that the gas is heated efiiciently and uniformly,and helps avoid damage to housing 5 from the intense heat generated. Themore dense and cooler portions of the gas are forced to the periphery ofthe opening by centrifugal force Hence, while the gas at the axis of theunit may be of extremely high temperatures, at the wall of passageway 6it is somewhat cooler.

After passing through the opening 6 the gas enters the mixing chamberand from thence leaves the unit through the outlet opening 17. Ofcourse, from the opening 17 the plasma may be used for any desiredpurpose. Generally there will be a suitable nozzle, test chamber orother arrangement attached to the outlet 17. Also, in the mixing chamber315 it is possible to inject various materials so that the exhaustplasma may have different compositions. Chemical synthesis effects maybe achieved in this manner.

The modification of FIGURE 3 possesses the same general characteristicas the unit of FIGURES 1 and 2. A central portion of the plasmagenerator, however, is made up of a tubular metallic section 38 which isof hollow nature to define, a cooling water passageway 39. The coolantmay enter the passageway through the inlet opening 40 at one end andexhaust through outlet at at the opposite end. Again the arc 42. isforced to follow the relatively narrow passageway 33 intermediate theend chambers 44 and 35. The electrodes 46 and 47 again are of annularconstruction having inside diameters considerably greater than thediameter of the aperture 43. As illustrated in this version of theinvention a nozzle 49 is disposed at the outlet of mixing chamber 45providing a means to accelerate the plasma to supersonic velocities.This nozzle empties into a suitable chamber 50, which may comprise ahypherthermal tunnel or any other test chamber. A water passage- Way 51is provided around the length of the nozzle, with inlet and outlets 52and 53 respectfully being provided for the coolant.

As illustrated in the arrangement of FIGURE 3 the current flows inopposite directions through the coils 55 and 56. Hence, the coils buckeach other and there is an independent flux field 57 around the coil 56,and a second separate flux field 58 at the coil 56. Consequently,because the terminal portions of the arc travel in opposite directionsas they cut their fiux fields, the rotational effeet at the footpointsof the are 42 is the same, causing the arc footpoints to rotate in thesame direction rather than in opposite directions as in the previouslydescribed embodiment. Hence, it is quite simple to cause the ends of thearc to rotate either in the same manner or oppositely. Of course, thecoils 27 and 32 could be made in bucking relationship in so far as theversion of FIGURE 1 is concerned, and similarly the coils 55 and 56could act together so as to rotate the ends of the arc 42 in oppositedirections.

From the foregoing it can be seen that I have provided a relativelysimple plasma generator capable of producing plasma of a high heatcontent despite the presence of high pressure gas at the inlet to theunit. Consequently, the device is versatile and can be used for avariety of purposes. The unit is efiicient as a result of the physicalconfinement of the arc to the relative narrow passage through the centerof the unit which also transmits the gas that is circulated through theplasma generator. This, with the combination of the rotating footpointsof the arc, gives the unit a long life, a stable arc and predictableperformance.

The foregoing detailed description is to be clearly understood as givenby way of illustration and example only. The spirit and scope of thisinvention being limited solely by the appended claims.

I claim:

1. A plasma generating device comprising a housing,

said housing having a first chamber at one end,

and a second chamber at the other end,

said second chamber having an outlet openopening, said housing having arelatively small aperture interconnecting said chambers, a firstelectrode in said first chamber, a second electrode in said secondchamber,

said electrodes being of annular configuration, and

having inside diameters greater than the diameter of said relativelysmall aperture,

a first coil circumscribing said first chamber adjacent said firstelectrode,

a second coil circumscribing said second chamber adjacent said secondelectrode,

means for introducing gas into said first chamber in a rotationalpattern,

and means for supplying current to said coils and said electrodes.

2. A device as recited in claim 1 in which said means for introducinggas into said first chamber adjacent said first electrode includes meansfor rotating said gas in the same direction as the rotation imparted bysaid first coil to the footpoint of said are at said first electrode,

whereby said gas and said first coil cooperate in rotating saidfootpoint at said first electrode.

3. A plasma generating device comprising a first chamber, a secondchamber, an elongated passageway interconnecting said chambers,

said passageway and said chambers being substantially circular in crosssection, said second chamber having an outlet opening opposite from saidpassageway, a first annular electrode in said first chamber adjacentsaid passageway, 21 second annular electrode in said second chamberadjacent said passageway,

said electrodes being of greater diameter than the diameter of saidpassageway, means for introducing gas into said first chamber in arotational flow pattern, a first coil circumscribing said first chamberadjacent said first electrode, a second coil circumscribing said secondchamber adjacent said second electrode, means for supplying current tosaid electrodes and to said coils,

for producing an are between said electrodes confined by saidpassageway, and for imposing a magnetic rotational force on ReferencesCited UNITED STATES PATENTS 2,945,119 7/1960 Blackman 313-231 X3,027,446 3/1962 Browning 2-1975 3,075,115 1/1963 Flowers 315111 X3,097,292 7/1963 Kugler 219-l21 3,140,421 7/1964 Spongberg 313231 XFOREIGN PATENTS 852,054 10/1960 Great Britain.

DAVID J. GALVIN, Primary Examiner.

GEORGE N. WESTBY, JAMES W. LAWRENCE,

Examiners.

S. D. SCI-ILOSSER, Assistant Eraminer.

3. A PLASMA GENERATING DEVICE COMPRISING A FIRST CHAMBER, A SECONDCHAMBER, AN ELONGATED PASSAGEWAY INTERCONNECTING SAID CHAMBERS, SAIDPASSAGEWAY AND SAID CHAMBERS BEING SUBSTANTIALLY CIRCULAR IN CROSSSECTION, SAID SECOND CHAMBER HAVING AN OUTLET OPENING OPPOSITE FROM SAIDPASSAGEWAY, A FIRST ANNULAR ELECTRODE IN SAID FIRST CHAMBER ADJACENTSAID PASSAGEWAY, A SECOND ANNULAR ELECTRODE IN SAID SECOND CHAMBERADJACENT SAID PASSAGEWAY, SAID ELECTRODES BEING OF GREATER DIAMETER THANTHE DIAMETER OF SAID PASSAGEWAY, MEANS FOR INTRODUCING GAS INTO SAIDFIRST CHAMBER IN A ROTATIONAL FLOW PATTERN, A FIRST COIL CIRCUMSCRIBINGSAID FIRST CHAMBER ADJACENT SAID FIRST ELECTRODE, A SECOND COILCIRCUMSCRIBING SAID SECOND CHAMBER ADJACENT SAID SECOND ELECTRODE,